Metallocene catalysts are widely used to produce polyolefin polymers such as polyethylene polymers. They have provided efficient processes and a variety of new and improved polymers. While there are many advantages to using metallocene catalysts in olefin polymerizations, there remain significant challenges. For example, metallocene catalysts, in particular supported metallocene catalysts, may have poor flowability and the catalyst particles tend to adhere to surfaces or form agglomerates. Adding other reagents to the catalyst composition such as commonly known antifouling agents or continuity additives/aids may compound the flowability issue. This causes practical problems in storing, transporting, and then delivering the dry catalyst into a polymerization reactor. There have been some attempts to address these issues.
In order to address reactor fouling problems caused by high activity metallocene catalysts, other additives such as carboxylate metal salts are often added to the catalyst, either separately or as part of the supported catalyst composition, such as in U.S. Pat. Nos. 6,300,436 and 5,283,278. However, such additives may compound the problem of the flowability of the catalyst.
The flowability problems associated with supported metallocene catalysts have been addressed in various ways. U.S. Pat. No. 5,795,838 is directed to metallocene halides, i.e., those metallocenes having at least one halide leaving group bound to the metal center. To address the flowability issue, the patent directs using a catalyst having certain levels of alkyl groups associated with the benzene-insoluble alkylalumoxanes used to form the supported catalyst compositions, and further, by prepolymerizing the catalyst prior to using it as a dry catalyst. U.S. Pat. Nos. 6,680,276 and 6,593,267 disclose heating the catalyst composition before or while combining it with the carboxylate metal salts. U.S. Pat. No. 6,660,815 discloses the use of a composition of carboxylate metal salt with a flow improver in combination with a polymerization catalyst to improve the flowability and bulk density of the catalyst. U.S. Pat. No. 7,323,526 discloses a supported catalyst composition having improved flow properties wherein the supported catalyst composition comprises an alkylalumoxane, a metallocene-alkyl, an inorganic oxide support having an average particle size of from 0.1 to 50 μm and is calcined at a temperature greater than 600° C., and optionally contains an antifoulant agent. PCT Publication WO 2009/088428, filed Jan. 7, 2008, discloses cooling the catalyst feeding system to maintain adequate catalyst flow.
In light of these attempts to address catalyst system flowability problems, challenges remain, especially at operating temperatures above about 25° C., and particularly above about 30° C.
Thus, it would be advantageous to have an improved catalyst system or composition that flows more easily at elevated temperatures, for example, above about 25° C. and 30° C., and, optionally, is capable of operating in a polymerization process continuously with enhanced reactor operability.